Digital communications may be carried out in a variety of ways. One method involves multiple frequency data transmission over analog communication lines. For example, discrete multi-tone (DMT) communications are employed by digital communication devices to transmit data over copper twisted-pair lines. Copper twisted-pair lines have been, and continue to be, widely employed in the telephone network, particularly for residential subscriber lines. As the need for high speed data communications to residential subscribers has increased, the implementation of DMT communications via digital subscriber line (DSL) modems has increased.
In the use of DMT communications, a DMT modulator modulates data onto a plurality of discrete tones. The modulated signal is then transmitted over the twisted-pair telephone lines to a point at which the data signal is recovered by a compatible receiver. A problem with DMT communications is that particular carrier frequencies selected for data modulation are within the frequency spectrum that may likewise be used for airborne signals, such as, for example, AM radio broadcasting. In particular, ingress of such airborne signals into a DMT communication line can interfere with the data transmission of the DMT modulators. Moreover, cross-talk from adjacent twisted pair lines carrying other DMT communications can cause interference.
The sources of interference cause a reduction in the effective data rate of a DMT communication link. Typically, the interference affects different tones of the multi-tone signal in different ways, depending upon the frequency characteristics of the interference. Noise from other sources can thus detract from the bit-carrying capacity or data rate on the various channels of the overall DMT signal. Because, among other things, noise and interference varies from DMT channel to DMT channel, some tones or channels on the same twisted pair line can carry more data than other tones. As a result, it is known to effect DMT communications by allocating different numbers of bits on each channel, based in part on the data rate capacity of each channel.
Effective allocation of bits on each channel requires, among other things, information as to the attenuation and noise present on each channel. To obtain such information, a test signal, typically a known pseudorandom signal, is transmitted over the line to a receiver using the entire multi-tone or multi-channel signal band. The receiver computes the impulse response over the signal band by computing the coherence between the received signal and its known content. In addition, the noise spectrum over the signal band is measured by measuring the received signal when the transmitter is silent. The impulse response and the noise spectrum then provide sufficient information to determine the per channel transfer function. With the channel transfer functions known, the highest achievable data rate, given a known power constraint, may be calculated for each channel. In general, a goal of DMT transmission is to achieve the highest data rate with the least amount of power.
A well-known method of allocating bits to channels in a DMT system is the so-called “water pouring” algorithm attributed to R. C. Gallager in “Information Theory and Reliable Communications”, (Wiley, 1968). The water pouring assumes well-defined noise and attenuation characteristics throughout the channel frequencies of a signal spectrum. These characteristics define a “terrain” of the spectrum. Allocating power is then similar to pouring a limited amount of water over the terrain with the same final level over the entire terrain, with the “water” depth at any point depending on the shape of the terrain. For the case of power allocation, the analogy translates to having equal power in all the bins that can hold power.
In this analogy, the “deepness” of each channel defines the maximum data rate for that channel. For example, frequency channels with the “deepest” water may transmit the most bits. Because power is evenly distributed, those channels that require less power to overcome noise and attenuation characteristics will be able to handle higher data rates.
A completely flat or even power distribution between the channels of a multi-channel signal is inherently inefficient, however, because of the relationship between power allocation and maximum data rate. Specifically, the relationship between available signal to noise power (i.e. water depth) and data rate is not smooth and continuous. In practical applications, data rates change in discrete steps. Thus, slight increases or decreases in power do not necessarily change the maximum possible data rate for a channel. Accordingly, in the water pouring algorithm, the equal allocation of power in the channels results in many or most channels having available signal to noise power that is somewhat in excess of that required for the closest possible data rate.
Existing algorithms seek to slightly adjust the otherwise even power distribution such that at least some of the excess power from some channels is reallocated to other channels that may be close to having enough power to support the next highest data rate. The result is that each channel that can carry data has roughly the same amount of power, however, with slight variations to account for the discrete power requirement steps for various data rates.
There are a number of factors that result in reduced performance of modems that employ the water pouring algorithm. One factor arises from the underlying assumption that the use of an equal amount of power provides the most efficient use of power over several channels. While this assumption is basically correct, it is also known that certain parts of the transmit power/data rate curve are inherently inefficient due to the characteristics of the coding schemes used to encode the data prior to DMT modulation. Thus, even when power is evenly distributed over several channels, a particular channel may be operating somewhat inefficiently at its particular power level.
There is a need, therefore, for a method and/or arrangement for allocating power and data bits to channels of a multifrequency communication circuit that has improved efficiency. There is further a need for a method and/or arrangement for allocating power that takes into account the efficiencies of the encoding schemes that are employed.